1. Field of the Invention
This invention relates to a method and apparatus for decreasing undesirable emissions in the exhaust of an internal combustion engine, and more particularly, to a new and improved method and apparatus for selectively introducing controlled quantities of oxygen and/or nitrogen enriched air into the intake of an internal combustion engine to thereby decrease the undesirable emissions that are present in the exhaust of the internal combustion engine.
2. Background of the Invention
Compression ignition (diesel) engines typically have high exhaust emissions, such as particulates, visible smoke, and oxides of nitrogen (NO.sub.X). Environmental Protection Agency (EPA) emissions standards require simultaneous reduction of NO.sub.X and particulate emissions to very low levels. This tends to be difficult to achieve because of the inherent tradeoffs between lowering both particulates and NO.sub.X emissions from a diesel engine. While it is possible in a diesel engine to reduce particulate emissions and to improve power density performance by using oxygen enriched intake air, such oxygen enriched intake air tends to also increase the amount of NO.sub.X in the exhaust being emitted from the diesel engine.
The type and amount of emissions in the exhaust of a spark ignition engine also is of concern. In order to meet the California Air Resources Board (CARB), Low Emission Vehicle (LEV) and Ultra Low Emission Vehicle (ULEV) standards, substantial reductions are required for carbon monoxide (CO) and hydrocarbon (HC) emissions from a spark ignition engine during the cold phase of the federal test procedure cycle. Similarly, many light-duty passenger cars are required to decrease these emissions to comply with United States EPA's Tier-II (year 2004) standards. This growing concern over start-up/cold-phase emissions has led to various attempts to develop new emissions treatment techniques that decrease the HC and CO levels in the exhaust emissions.
Attempts have been made by others to decrease cold-phase (i.e., the first 505 seconds of federal test procedure driving cycle, as specified in the Code of Federal Regulations, Title 40, Part 86, Subpart 8, revised 1993) emissions by after-treatment methods. These methods can be grouped as follows: (1) thermal management of the catalytic converter, including low-mass manifolds, double-walled exhaust pipes, electrically heated catalysts, exhaust-gas burners, exhaust-gas igniters, and insulated converters (with vacuum or refractory material); (2) placement of the converter closer to the exhaust manifold; and (3) management of the interaction between the hydrocarbons and the catalyst, using hydrocarbon adsorbent or traps in the exhaust. However, durability, fuel penalty, additional capital costs, unwanted heat in the engine compartment, and the complexity of these systems limit their application in vehicles.
An alternative to such after-treatment methods is to control the emissions at the source itself (i.e., during combustion). One type of in-cylinder emission control is to introduce oxygen enriched air instead of ambient air to the air intake of the engine. Use of such oxygen enriched air can potentially decrease CO and HC emissions from a spark ignition engine, even during start-up and warming up periods, because oxygen enrichment of the intake air decreases the emissions from the engine rapidly (even when the engine is cold). In fact, it helps to minimize the catalytic converter limitations during the cold phase operation of the engine and should improve catalytic converter efficiency. This method has the advantage of fewer add-on components, of lesser mechanical complexities, of not altering the fuel economy of the engine, and of an easier to modify system (the air intake system is easier to modify than the exhaust system).
Even though the oxygen enrichment of the intake air in spark ignition engine powered vehicles results in the lowering of cold-phase HC and CO emissions in the exhaust of the vehicle, it tends to result in an increase in NO.sub.X in the exhaust due to higher combustion temperatures. To some extent, the increase in NO.sub.X in the emissions can be offset by NO.sub.X control technologies that can remove nitrogen oxides from the emissions. These technologies include lean NO.sub.X catalysts and the injection into the exhaust gases of monatomic-nitrogen induced by a pulse arc (see, for example, U.S. Pat. No. 5,526,641 that is assigned to the same assignee of record as the present application).
In the case of both diesel and spark ignition engines, exhaust gas recirculation (EGR) systems have been used as one method of decreasing NO.sub.X emissions. When the gases from the EGR system are about 50% of the intake air, oxygen concentration is decreased from about 21% to about 14%. The decrease of NO.sub.X by the use of EGR systems tends to vary depending on the rate, temperature and water content of the EGR gases, injection timing, and air-fuel ratio of the intake to the engine. However, there are limits as to the amount of exhaust gases that can be reintroduced into the engine before power output and fuel economy are adversely affected. Such reintroduction of exhaust gases can also cause wear problems and oil contamination, particularly in the case of diesel engines where the recirculated gases include soot particles. The results obtained by using an EGR system are considered similar to the dilution of intake air with inert gases such as nitrogen because in both cases the intake oxygen concentration is decreased and the heat capacity of the intake air is increased. While the benefits of using EGR systems and nitrogen enriched air in the intake air of an engine to lower the NO.sub.X emissions are similar, the advantages of using nitrogen enriched air over recirculated gases from an EGR system are (i) the elimination of unwanted exhaust species (such as soot particles) being introduced into the engines intake, (ii) the elimination of heat exchangers to cool and control the temperature and water content of the exhaust gases to be recirculated, and (iii) the elimination of poor utilization of intake air (higher displacement of intake air).
Notwithstanding the advantages of introducing oxygen enriched air and/or nitrogen enriched air into the intake of an engine, the lack of an economical source of on-line oxygen and nitrogen has made it difficult to provide a practical application of the concept of providing oxygen and nitrogen enriched air to the air intake of the engine during various phases or time periods of operation of the engine. Relatively compact oxygen enrichment devices, such as selectively permeable membranes, have been used to provide oxygen enriched air to the air intake of an internal combustion engine (see U.S. Pat. No. 5,526,641 and U.S. patent application Ser. No. 08/598,029, filed on Feb. 7, 1996, both of which are assigned to the same assignee of record as the present application). Any such system requires a relatively simple, compact mechanical system driven by the engine itself that will economically extract oxygen and nitrogen from the ambient air and provide oxygen enriched and/or nitrogen enriched air to the air intake of the engine during particular phases or time periods of operation of the engine.
Accordingly, it is an object of the present invention to provide a new and improved method and apparatus for decreasing particulate, HC, CO and NO.sub.X emissions in the exhaust of an internal combustion engine by selectively introducing oxygen enriched and/or nitrogen enriched air into the air intake of the engine vehicle during selected periods or phases of operation of the engine.
It is another object of the present invention to provide a new and improved method and apparatus for introducing oxygen enriched and/or nitrogen enriched air into the air intake of an internal combustion engine by diverting at least a portion of the intake air through a selectively permeable membrane so that oxygen enriched and/or nitrogen enriched air can be supplied to the engine intake manifold for selected periods of time during the operation of the engine and nitrogen enriched air can be supplied to a monatomic-nitrogen plasma generator that enables the chemical reduction of NO.sub.X in the exhaust of the engine.
It is yet another object of the present invention to provide a new and improved method and apparatus for introducing oxygen enriched and/or nitrogen enriched air into the air intake of an internal combustion engine by diverting at least a portion of the intake air through a selectively permeable membrane so that ambient air, oxygen enriched air and/or nitrogen enriched air can be selectively supplied to a mixing chamber and then from the mixing chamber to the engine intake manifold during selected periods of time of operation of the engine.